1. Field of the Invention
The present invention relates to a communication system and a gateway, and particularly to a communication system and a gateway which can be applied to a radio communication system to realize semi-duplex many-to-many multicasting service or a professional wireless system containing plural radio systems.
2. Description of the Related Art
In a professional wireless system such as a disaster prevention and administrative radio system and a fire radio system, it is indispensable that semi-duplex many-to-many multicasting service can be performed in which application data (voice, image, short message, etc.) can be exchanged among plural mobile stations. In recent years, on the assumption that the IP (Internet Protocol) network is used, it is socially requested to configure a professional wireless system. Hitherto, professional wireless systems are independently managed for the respective organizations. However, it is requested that the interconnection between the systems can be easily realized by the IP so that the organizations can communicate to each other the time of disaster or the like. Especially, it is desired that the protocol of call control is based on the SIP (Session Initiation Protocol).
As a system in which the semi-duplex many-to-many multicasting service is performed among plural mobile stations on the IP network, there is a method disclosed in non-patent document 1. The method disclosed in non-patent document 1 is a method based on PoC (Push to talk Over Cellular) determined by OMA (Open Mobile Alliance) which is a standard-setting organization. The PoC is the typical specification for performing the semi-duplex many-to-many multicasting service among mobile stations capable of performing IP communication. According to this, a PoC system includes a PoC server, mobile stations called PoC clients, and a SIP server. The PoC server is a press-talk server which performs call connection to plural mobile stations as objects of the multicasting service, and delivers application data. Besides, the PoC server assigns and notifies the floor to the mobile station in order to determine the sender of application data. The PoC server and the mobile station exchanges call control signals of the SIP through the SIP server.
Non-patent document 1: System Development of Push Talk Service, NTT DoCoMo Technical Journal Vol. 13, No. 4, p 6-13